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**2016/240**

**Title:**

On Error Distributions in Ring-based LWE

**Authors:**
Wouter Castryck, Ilia Iliashenko, Frederik Vercauteren

**Abstract:**

Since its introduction in 2010 by Lyubashevsky, Peikert and Regev, the Ring Learning With Errors problem (Ring-LWE) has become a popular building block for cryptographic primitives, due to its great versatility and its hardness proof consisting of a (quantum) reduction from ideal lattice problems. But for a given modulus q and degree n number field K, generating Ring-LWE samples can be perceived as cumbersome, because the secret keys have to be taken from the reduction mod q of a certain fractional ideal \mathcal{O}_K^\vee \subset K called the codifferent or `dualâ€™, rather than from the ring of integers \mathcal{O}_K itself. This has led to various non-dual variants of Ring-LWE, in which one compensates for the non-duality by scaling up the errors. We give a comparison of these versions, and revisit some unfortunate choices that have been made in the recent literature, one of which is scaling up by |\Delta_K|^{1/2n} with \Delta_K the discriminant of K. As a main result, we provide for any \varepsilon > 0 a family of number fields K for which this variant of Ring-LWE can be broken easily as soon as the errors are scaled up by |\Delta_K|^{(1-\varepsilon)/n}.

**ePrint:**
https://eprint.iacr.org/2016/240

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